A normal spinal disk is a cylindrical weight-bearing fibrous structure with a non-compressible viscous center. Due to its ability to deform, the spinal disk not only supports normal functional loads of the human body (e.g., load bearing) but also cushions and evenly distributes the pressures or stresses applied with body movement and positioning (e.g., load sharing). The spinal disk articulates between two bony vertebrae, one vertebra above the disk and one vertebra below the disk, through large surface area interfaces known as endplates. An endplate is a thin (e.g., 1 mm-3 mm) approximately round plate (e.g., 2 cm to 4 cm in diameter) of dense bone and cartilage accounting for a majority of the vertebral weight-bearing capacity.
The spinal disk represents just one of the components defining motion or articulation between vertebrae. The other components are two symmetric facet joints that form a triangular arrangement with the spinal disk being disposed in front. The spinal disk functions as a substantial hydraulic spacer between the vertebrae. Vertical loads with flexion, extension, lateral bending or rotation movements applied to the spine cause the spinal disk to deform and create secondary movement between the vertebrae. Movement across the spinal disk is coupled to the movement of the symmetric facet joints, which function similarly to classical joints with relative translation between two opposing surfaces.
The articulations between the vertebrae, including the foregoing spinal disk and facet joints, frequently deteriorate with age or trauma and become a source of pain. Spinal disk deterioration causes the spinal disk to lose its normal consistency and volume, facilitating the spinal disk to collapse and causing abnormally painful motion within the anterior spinal column. Furthermore, the abnormal motion across the spinal disk increases the stresses on the facet joints and accelerates degeneration of the facet joints.
Historically, surgical treatment of spinal disk disorders required fusion or elimination of movement across an abnormal spinal disk. This has been accomplished by allowing bone to grow between adjacent vertebrae and through the disk space of the abnormal spinal disk. Although fusion generally relieved the source of pain, fusion however did not restore normal movement of the fused spinal motion segment. Invariably, fusion eliminates a range of motion in the fused spinal motion segment, limits overall spinal range of motion and places abnormal pressures or stresses on other non-fused normal spinal motion segments with body movement and positioning. Thus, the abnormal pressures or stresses caused by fusion may further accelerate the degeneration of the foregoing articulations between normal vertebrae.
A new class of restorative or motion preserving spinal devices has been introduced to overcome the foregoing limitations of fusion. These motion preserving spinal devices aim to restore and maintain spinal disk height while approximating a range of motion and function of the normal spinal disk.
The motion preserving spinal devices include artificial spinal disks that generally have rigid movably coupled components and ball-socket articulation. More specifically, the artificial spinal disks function through direct contact and movement between two opposing surfaces, usually metal or plastic. The ball-socket articulation (among other mechanical contact points) produce hazardous debris and cannot reproduce adequately normal spinal disk deformation or its load sharing capacity. The mechanical contact points (including ball-socket articulation) of the artificial spinal disk components wear with repetitive motion and produce debris which may induce scarring, toxicity and bone absorption. The scarring may be extensive with the potential for neural injury and bone loss. Certain debris (e.g., nickel) accumulates in the body and may cause systemic toxicity. The mechanical wear further may cause breakdown of artificial spinal disk components and resultant painful malfunction of the artificial spinal disk. Furthermore, non-constrained components may extrude into the abdomen with disastrous consequences.
One way of approximating the motion of the normal spinal disk has been to implement a floating center of movement. However, computer simulations using finite element analysis of currently available artificial spinal disks have shown excessive or abnormal motion at spinal disk interfaces, particularly in extension, when compared to the normal spinal disk. These data have been confirmed by biomechanical testing of the artificial spinal disks in cadavers. The abnormal motion at the artificial disk interfaces wears artificial spinal disk components and puts abnormal strain on the facet joints of the vertebrae, significantly accelerating painful and debilitating degeneration of the vertebrae.
While the new class of restorative or motion preserving spinal devices aims to solve the limitations of fusion, the foregoing abnormal strain on the facet joints, the wear of the artificial spinal disk with resultant debris and possible failure of the artificial spinal disk increase painful and debilitating degeneration of the vertebrae and may further in the case of extrusion present real dangers to one's health.